1. Field of the Invention
The present invention relates to the verification of electrode integrity in a measuring instrument, particularly an instrument having a high input impedance, such as an electromagnetic flowmeter or a pH or redox or other chemical (or bio-chemical) probe-type meter.
2. Description of the Related Art
It has been proposed to measure resistance between electrodes by injecting a constant current from a constant current source. This method works, but has the drawback that a complex circuit is required to inject the constant current.
EP 336 615 discloses some alternative methods in which a signal is injected by means of a capacitor. A drawback of these methods is that there are constraints imposed on the timing of injection and sampling of the signal (specifically, it is essential for pulses to be injected at the beginning of each half cycle), and it may be difficult to obtain accurate results due to the somewhat irregular shape of the voltage waveform produced by the injected signal.
The invention aims to alleviate the above drawbacks and provide an arrangement in which a reliable measurement of electrode resistance can be obtained from a relatively simple circuit.
In a first aspect, the invention provides a method of obtaining an in situ measure of impedance (or conductivity) between potential sensing electrodes of a meter having a high input impedance, the method comprising applying a substantially linear voltage ramp waveform to a capacitor coupled to one of the electrodes to generate a substantially constant current, and deriving a measure of impedance (or conductivity) by comparing the potential developed across the electrodes while the constant current is flowing to the potential developed across the electrodes when no current, or a different current, is flowing.
Throughout this specification, reference is made to potential sensing electrodes; it will be appreciated that potential must be measured between two reference points. One of these reference points may be an earthing point or some contact with a solution rather than a conventional xe2x80x9celectrodexe2x80x9d of the meter in question (for example, a pH or reference electrode). In the specification and claims, the term xe2x80x9cpotential sensing electrodexe2x80x9d is intended to encompass any point from which a potential can be sensed; the invention extends to measurements between a single electrode and a solution using a suitable reference point.
An advantage of using a capacitor to inject the current is that complex switching arrangements are not required to isolate the electrodes from the impedance measuring circuitry when the potential across the electrodes is to be measured; conventional resistance measuring circuitry is liable to interfere with measurement of potential as the electrodes can typically source only a small current. Another advantage of the method is that, because the current is injected for a discrete period of time, the measurement has an opportunity to stabilise, enabling a reliable reading to be obtained without complex circuitry or correction required; this can be contrasted with pulsed measurement of impedance.
Preferably, a plurality of measures of potential are obtained while the current is injected. This enables measurement to be averaged over a period of time, which may enable noise to be cancelled or readings to be averaged to provide greater accuracy, and can provide surprisingly improved accuracy as compared to pulsed single measurements. Surprisingly, if only two readings are taken during the duration of current injection, significantly greater accuracy and consistency of results may be obtained, as the measurement is less susceptible to transients.
Preferably a measure of potential is obtained after a predetermined (relatively short) delay after commencement of injection of current. This enables the apparatus to settle, and allows any (small) stray capacitances between the electrodes to be effectively charged.
It will be understood that by substantially linear is meant that, within the limits of experimental accuracy required, the current generated by the ramp is within a desired tolerance range while the measurement is made. High input impedance is meant an impedance sufficient to ensure that the potential measured across the electrodes is not significantly (within the limits of experimental accuracy required) affected by connection of the potential sensing circuitry; ideally the impedance will be at least 1M ohm, and typically 10M ohms, 100M ohms or higher. By small current is meant a current that is typically at most a few micro amps,but may be many orders of magnitude lower (less than 1 micro amp, less than 100 nA, 10 nA or even less).
The method preferably further comprises obtaining a measure of the potential across the electrodes to derive therefrom a measure of a physical property related to the potential, said measure of potential being obtained by potential measuring means having a high input impedance without disconnecting said capacitor from said electrode.
In one preferred application, the physical property is flow rate, the method being employed in an electromagnetic flowmeter. In another preferred application, the physical property is pH, the method being employed in a pH meter. In a similar manner to a pH meter, other chemical (or bio-chemical) conditions may be sensed, for example in a redox potential meter. In both cases, the measure of impedance can be used to detect conditions such as fouled or faulty electrodes, broken wiring, absence of fluid and the like.
The method preferably includes comparing the measure of impedance to at least one threshold, and signalling at least one suspected fault condition in dependence on the results of the comparison.
The ramp waveform may be generated by any of a number of conventional ramp voltage generators.
A preferred arrangement which has the benefit of being simple and cost effective to implement is to couple the input of the capacitor to the junction between a series resistor-capacitor circuit, the potential across the combination being switched between two potentials, the time constant of the circuit being greater than the measurement period.
Alternatively, a more complex ramp synthesiser, for example based on a digital to analogue convertor or conventional linear ramp generator may be used.
The invention extends to both method and apparatus aspects, and it will be appreciated that preferred features of the method may be applied to the apparatus, and vice versa.
In a first apparatus aspect, the invention provides sensing apparatus for a meter arranged to derive a measure of a physical property from a measure of potential across sensing electrodes, the sensing circuit comprising a potential measuring circuit having a high input impedance and inputs arranged for connection to the electrodes and means for obtaining a measure of the impedance of the electrodes comprising a capacitor coupled between one of said inputs and means for generating a substantially linear ramp voltage so that the ramp voltage generates a substantially constant current through the electrode impedance, the apparatus further including means for deriving a measure of impedance based on the difference in potential across the electrodes when the substantially constant current is supplied and when no current, or a different value of current is supplied.
Preferably, the apparatus includes a capacitor coupled to each input; in this way, the absolute potential of the electrodes relative to the sensing circuit can be left floating.
The apparatus preferably further includes means for comparing the measure of impedance to at least one threshold and means for signalling a suspected fault condition based on the results of the comparison.
The sensing circuit may be employed in a pH meter including a pH sensing electrode, th sensing apparatus and an output circuit arranged to provide a calibrated measure of pH based on the measured electrode potential. The output circuit may comprise additional circuitry supplied with the output of the potential measuring circuit, or may be integrated therewith, the potential measuring circuit providing an appropriately scaled output signal.
The calibrated measure need not be individually calibrated for a particular apparatus, but may be scaled appropriately based on a general relationship between measured potential and pH for meters of a similar design.
As mentioned above, another preferred application of the invention is in an electromagnetic flowmeter, and in particular in a flowmeter having low power consumption, such as a battery-powered flowmeter.
In such an application, the apparatus preferably further includes control means arranged to control application of current to field generating coils of the flowmeter and to control application of the ramp voltage to the capacitor to enable measurements of both flow and electrode impedance to be obtained.
Preferably, also, the apparatus is arranged to apply a magnetic field to the fluid while said substantially constant electrode current is applied, and to obtain samples of electrode potential in the presence of the magnetic field and of the substantially constant current, and in the presence of the substantially constant current alone. This may enable determination of both flowrate and electrode impedance, without requiring prolonged application of an electromagnetic field; in this way, power consumption may be reduced, as application of the magnetic field typically requires significantly more power than application of the substantially constant electrode current.
The method may be adapted for use with a flowmeter.
Most preferably, the method comprises applying a pulsed magnetic field to the fluid during application of said substantially constant electrode current and obtaining successive first, second and third values of electrode potential respectively before, during, and after application of the pulsed magnetic field, all during application of the substantially constant electrode current. In this way, variations in the substantially constant current can be compensated for by averaging. In addition, the inventors have found that, unexpectedly, better results may be obtained if a relatively short magnetic pulse is applied than if a short electrical pulse is applied. Furthermore, application of the magnetic field generally requires substantially more power than application of the electrode current, so application of a short magnetic field and a longer electric field may reduce power consumption.
To improve results further, further values of the electrode potential may be obtained in the absence of said substantially constant electrode current, preferably at least one further value in the absence of a magnetic field, and another further value in the presence of a magnetic field. Preferably, magnetic pulses of alternating plurality are employed; this may reduce hysteresis effects. In addition (independently) electrode currents of alternating polarity may be employed; this may reduce the effects of polarisation. In both cases, references to alternating polarity (particularly in the case of currents of alternating polarity) may be extended to include groups of pulses of alternating polarity. For example, a sequence of pulse elements (which may have varying magnitudes or signs) may be followed by a similar sequence in which the polarity of each or at least the majority of the pulse elements is reversed; this may still inhibit long-term polarisation without every consecutive element alternating in polarity.
The invention may be employed to provide an EM flowmeter with an xe2x80x98empty pipe detectorxe2x80x99, which is required to ensure the flowrate output is controlled, usually driven down scale, under this empty or partially full pipe condition.
The invention may also be applied to flowmeters having a permanent magnet to generate a magnetic field.